A liquid crystal display mode which has meanwhile found widespread interest and commercial use is the so-called PS (“polymer sustained”) or PSA (“polymer sustained alignment”) mode, for which the term “polymer stabilised” is also occasionally used. In PSA displays an LC medium is used that contains an LC mixture (hereinafter also referred to as “host mixture”) and a small amount, typically <1% by weight, for example 0.2 to 0.4% by weight, of one or more polymerisable compounds, preferably polymerisable monomeric compounds. After filling the LC medium into the display, the polymerisable compounds are polymerised or crosslinked in situ, usually by UV photopolymerisation, optionally while a voltage is applied to the electrodes of the display. The polymerisation is carried out at a temperature where the LC medium exhibits a liquid crystal phase, usually at room temperature. The addition of polymerisable mesogenic or liquid-crystalline compounds, also known as reactive mesogens or “RMs”, to the LC host mixture has proven particularly suitable.
The PS(A) mode is meanwhile used in various conventional LC display types. Thus, for example, PS-VA (“vertically aligned”), PS-OCB (“optically compensated bend”), PS-IPS (“in-plane switching”), PS-FFS (“fringe-field switching”), PS-UB-FFS (“Ultra Brightness FFS) and PS-TN (“twisted nematic”) displays are known. The polymerisation of the RMs preferably takes place with an applied voltage in the case of PS-VA and PS-OCB displays, and with or without, preferably without, an applied voltage in the case of PS-IPS displays. As a result a pretilt angle of the LC molecules is generated in the display cell. In case of PS-OCB displays, for example, it is possible for the bend structure to be stabilised so that an offset voltage is unnecessary or can be reduced. In case of PS-VA displays, the pretilt has a positive effect on the response times. For PS-VA displays, a standard MVA (“multidomain VA”) or PVA (“patterned VA”) pixel and electrode layout can be used. It is also possible to use only one structured electrode without protrusions, which significantly simplifies production and improves contrast and transparency.
Furthermore, the so-called posi-VA mode (“positive VA”) has proven to be particularly suitable. Like in conventional VA and PS-VA displays, the initial orientation of the LC molecules in posi-VA displays is homeotropic, i.e. substantially perpendicular to the substrates, in the initial state when no voltage is applied. However, in contrast to conventional VA and PS-VA displays, in posi-VA displays LC media with positive dielectric anisotropy are used. Like in IPS and PS-IPS displays, the two electrodes in posi-VA displays are arranged only on one of the two substrates, and preferably exhibit intermeshed, comb-shaped (interdigital) structures. Upon application of a voltage to the interdigital electrodes, which create an electrical field that is substantially parallel to the layer of the LC medium, the LC molecules are switched to an orientation substantially parallel to the substrates. In posi-VA displays, a polymer stabilisation by addition of RMs to the LC medium, which are then polymerised in the display, has also proven to be advantageous. Thereby a significant reduction of the switching times can be achieved.
PS-VA displays are described for example in EP1170626 A2, U.S. Pat. Nos. 6,861,107, 7,169,449, US2004/0191428A1, US2006/0066793A1 and US2006/0103804A1. PS-OCB displays are described for example in T.-J-Chen et al., Jpn. J. Appl. Phys. 45, 2006, 2702-2704 and S. H. Kim, L.-C-Chien, Jpn. J. Appl. Phys. 43, 2004, 7643-7647. PS-IPS displays are described for example in U.S. Pat. No. 6,177,972 and Appl. Phys. Lett. 1999, 75(21), 3264. PS-TN displays are described for example in Optics Express 2004, 12(7), 1221.
PSA displays can be operated as either active-matrix (AM) or passive-matrix (PM) displays. In case of AM displays individual pixels are usually addressed by integrated, non-linear active elements like for example transistors (such as thin-film transistors or “TFTs”), whereas in PM displays individual pixels are usually addressed by the multiplex method as known from prior art.
A PSA display may also comprise an alignment layer on one or both of the substrates forming the display cell. The alignment layer is usually applied on the electrodes (in case such electrodes are present) such that it is in contact with the LC medium and induces initial alignment of the LC molecules. The alignment layer may comprise or consist of, for example, a polyimide, which may also be rubbed or prepared by a photoalignment method.
In particular for monitor and especially TV applications optimisation of the response times, but also of the contrast and luminance (and thus transmission) of the LC display is still desired. The PSA method can provide significant advantages here. Especially in case of PS-VA, PS-IPS, PS-FFS and PS-posi-VA displays, a shortening of the response times, which correlate with a measurable pretilt in test cells, can be achieved without significant adverse effects on other parameters.
Prior art has suggested biphenyl diacrylates or dimethacrylates, which are optionally fluorinated, as RMs for use in PSA displays
However, the problem arises that not all combinations of LC host mixture and RM(s) are suitable for use in PSA displays because, for example, only inadequate tilt angles or no tilt angles at all could be generated or because, for example, the voltage holding ratio (VHR) is inadequate for TFT display applications. In addition, it has been found that the LC mixtures and RMs known from prior art still have some disadvantages when used in PSA displays. Thus, not every known RM which is soluble in the LC host mixture is suitable for use in PSA displays. In addition, it is often difficult to find a suitable selection criterion for the RM besides direct measurement of the pretilt in the PSA display. The choice of suitable RMs becomes even smaller if UV photopolymerisation without the addition of photoinitiators is desired, which is advantageous for certain applications.
In addition, the selected combination of LC host mixture/RM should have a low rotational viscosity and good electrical properties, in particular a high VHR. In PSA displays, a high VHR after irradiation with UV light is particularly important, because UV exposure does not only occur as normal exposure during operation of the finished display, but is also a necessary part of the display production process.
In particular, it is desirable to have available improved materials for PSA displays which produce a particularly small pretilt angle. Preferred materials are those which, compared to prior art materials, can generate a lower pretilt angle after the same exposure time, and/or can generate at least the same pretilt angle after a shorter exposure time. This would allow to reduce the display production time (“tact time”) and production costs.
A further problem in the production of PSA displays is the presence and removal of residual amounts of unpolymerised RMs after the polymerisation step that is necessary for generation of the pretilt angle in the display. Unreacted RMs may adversely affect the properties of the display, for example by polymerising in an uncontrolled manner during display operation.
Thus, the PSA displays known from prior art often exhibit the undesired effect of so-called “image sticking” or “image burn”, i.e. the image produced in the LC display by temporary addressing of individual pixels still remains visible even after the electric field in these pixels has been switched off, or after other pixels have been addressed.
Image sticking can occur for example if LC host mixtures having a low VHR are used. The UV component of daylight or the display backlight can cause undesired decomposition reactions of the LC molecules and initiate the production of ionic or free-radical impurities. These can accumulate in particular at the electrodes or the alignment layers, where they reduce the effective applied voltage. This effect can also be observed in conventional LC displays without a polymer component.
An additional image sticking effect caused by the presence of unpolymerised RMs is often observed in PSA displays. Uncontrolled polymerisation of the residual RMs is initiated by UV light from the environment or the backlight. In the switched display areas, this changes the tilt angle after a number of addressing cycles. As a result, a change in transmission in the switched areas may occur, while it remains unchanged in the unswitched areas.
During production of the PSA display it is therefore desirable that polymerisation of the RMs proceeds as completely as possible and the presence of unpolymerised RMs in the display can be excluded or reduced to a minimum. Thus, RMs and LC host mixtures are required which enable or support quick and complete polymerisation of the RMs. In addition, a controlled reaction of the residual RM amounts is desirable. This could be achieved by providing improved RMs that polymerise quicker and more effectively than the RMs of prior art.
A further problem that has been observed in the operation of PSA displays is the stability of the pretilt angle. Thus, it was observed that the pretilt angle, which is generated during display manufacture by polymerising the RMs, does not remain constant but can deteriorate after the display was subjected to voltage stress during display operation. This can negatively affect the display performance, e.g. by increasing the black state transmission and hence lowering the contrast.
Another problem to be solved is that the RMs of prior art do often have high melting points, and do only show limited solubility in many commonly used LC mixtures. As a result the RMs tend to spontaneously crystallise out of the LC mixture. In addition, the risk of spontaneous polymerisation prevents that the LC host mixture can be warmed in order to better dissolve the RMs, so that a high solubility even at room temperature is required. In addition, there is a risk of phase separation, for example when filling the LC medium into the LC display (chromatography effect), which may greatly impair the homogeneity of the display. This is further aggravated by the fact that the LC media are usually filled in the display at low temperatures in order to reduce the risk of spontaneous polymerisation (see above), which in turn has an adverse effect on the solubility.
Another problem observed in prior art is that the use of conventional LC media in LC displays, including but not limited to displays of the PSA type, often leads to the occurrence of mura in the display, especially when the LC medium is filled into the display by the one drop filling (ODF) method. This phenomenon is also known as “ODF mura” or “ODF drop mura”. It is therefore desirable to provide LC media which lead to reduced ODF mura.
Presently ODF is the preferred method to apply an LC medium to an AM LCD. The ODF process is exemplarily and schematically illustrated in FIG. 1. In a first step (a) an array of droplets (2) of the LC medium is dispensed on one of the LCD glass substrates (1), where a sealant material is provided in the boundary region (3) between the LC droplets (2) and the edges of the substrate (1). In a second step (b), in a vacuum assembly station the counter glass substrate (4) is coupled and fixed to the first substrate (1), thus forcing the LC droplets (2) to spread and form a continuous layer between the two substrates (1, 4). Because of the different flow dynamics during drop dispensing and substrate coupling, various types of non-uniformity in brightness, so-called mura, can be observed as an identifier of the LC dispensing and assembly process.
In particular, if the display mode is a VA type mode, like MVA, PVA or PS-VA, the imprint of the droplets is visible after fabrication because the droplets are not spreading evenly over the entire display area. In conventional VA modes, like for example MVA or PVA, the non-uniformity generally disappears with time passing. In PSA displays, however, the non-uniformity remains, and the droplet imprint is “fixed” by the polymerisation process. The usual way to minimize ODF drop mura is by optimizing the process conditions, the panel design and/or the LC materials used, such as the LC host mixture or the RM contained therein, for example by reducing the RM concentration.
However, a breakout of ODF mura has been observed even after the tact time of the polymerisation process was reduced by increasing the intensity of the short-wave part of the irradiation spectrum. Also, a reduction of the RM concentration can negatively influence the pretilt angle generation.
There is thus still a great demand for PSA displays, and for LC media and polymerisable compounds for use in PSA displays, which do not show the drawbacks as described above, or only do so to a small extent, and have improved properties.
In particular, there is a great demand for PSA displays, and LC mixtures and RMs for use in such PSA displays, which enable a high specific resistance at the same time as a large working-temperature range, short response times, even at low temperatures, a low threshold voltage, a low pretilt angle, a multiplicity of grey shades, high contrast and a broad viewing angle, high reliability and high values of the VHR after UV exposure, and, in case of the RMs, have low melting points and a high solubility in the LC host mixture. In PSA displays for mobile applications, it is especially desired to have available LC media that show low threshold voltage and high birefringence.
Furthermore, there is a great demand for means and methods for reducing the ODF mura in PSA displays, especially in PS-VA displays.
The invention is based on the object of providing improved methods for manufacturing PSA displays and improved materials used therein, like RMs, LC host mixtures, and LC media comprising the same, which do not have the disadvantages indicated above or do so to a reduced extent.
In particular, an object of the invention is to solve the problem of reducing ODF mura in PSA displays which is caused in the display manufacturing process.
Another object of the invention is to provide LC media for use in PSA displays, which enable very high specific resistance values, high VHR values, high reliability, low threshold voltages, short response times, high birefringence, show good UV absorption especially at longer wavelengths, allow quick and complete polymerisation of the RMs contained therein, allow the generation of a low pretilt angle as quickly as possible, enable a high stability of the pretilt even after longer time and/or after UV exposure, reduce or prevent the occurrence of image sticking in the display, and reduce or prevent the occurrence of ODF mura in the display.
The above objects have been achieved in accordance with the present invention by the methods and materials as described and claimed in the present application.
It has surprisingly been found that at least some of the above-mentioned problems can be solved by using an LC medium comprising a polymerisable component and an LC host mixture as disclosed and claimed hereinafter.
In particular it has surprisingly been found that, when adding to the LC medium which contains a first RM a little amount of a second RM with faster polymerisation speed than the first RM, the ODF mura level decreases. By adding a second RM that reacts faster than the first RM at the given UV excitation spectrum the level of the ODF drop mura is reduced. The concentration of the second RM should be sufficiently low not to have an impact on the overall performance of the LC display. A suitable and preferred concentration range for the second RM in the LC medium is for example around 100 ppm, while a suitable and preferred concentration range for the first RM in the LC medium is for example in the range of 0.2 to 0.5%.
In addition, the use of an LC medium as disclosed and claimed hereinafter in PSA displays enables to achieve high VHR, high UV absorption, quick and complete polymerisation, and quick and strong tilt angle generation.
Moreover, the use of LC media according to the present invention facilitates a quick and complete UV-photopolymerisation reaction in particular at low UV energy and/or longer UV wavelengths in the range from 300-380 nm and especially above 340 nm, which are considerable advantages for the display manufacturing process. Besides, the use of LC media according to the present invention allows a fast generation of large and stable pretilt angles, reduces image sticking and ODF mura in the display, leads to a high VHR value after UV photopolymerisation, and enables to achieve fast response times, a low threshold voltage and a high birefringence.